Displacement measuring interferometers (“DMIs”) are well known in the art, and have been used to measure small displacements and lengths to high levels of accuracy and resolution for several decades. Among such devices, helium-neon displacement measuring laser interferometers have enjoyed relatively widespread application owing to their high degree of stability and monochromaticity. Interferometers require careful alignment of mirrors that must be sustained over extended periods of time, however, which can present considerable practical difficulties.
A double-pass DMI may be rendered partially insensitive to mirror misalignments and thermal effects by double-passing each arm of the interferometer and incorporating a means for inverting the wavefronts between passes. See, for example, “A Double-Passed Michelson Interferometer” by S. J. Bennett in Optics Communications, Volume 4, number 6, February/March, 1972, where double-passing is achieved using a polarized beam-splitter, two quarter-wave plates and a cube-corner reflector that serves as an inverting component, the entirety of the foregoing paper by Bennett hereby being incorporated by reference herein. In consequence of their commercial viability, robustness, stability and accuracy, double-pass displacement measuring interferometers find relatively common use in high accuracy displacement measurements.
Despite the many advances that have been made in the field of double-pass interferometers and DMIs generally, however, measurement errors and inaccuracies persist. Among the factors contributing to such errors and inaccuracies is relative beam displacement (“RBD”). See, for example, “Wavefront Metrology Errors ” by Eric Johnstone et al. in 4th International Conference of the European Society for Precision Engineering and Nanotechnology (uespen) May-June 2004 Glasgow UK page 348-349 and “Recent Advances in Displacement Measuring Interferometry” by Norman Brobroff in Meas. Sci. Technol. 4(1993)907-926, where some of these factors are discussed in detail, the entirety of the foregoing paper by Broboff hereby being incorporated by reference herein.
In a conventional monolithic dual-pass DMI, a laser source directs a beam towards an interferometer. A beam splitter in the interferometer splits the incoming beam and directs one portion of the beam into a reference arm and another portion of the beam into a measurement arm. The reference portion of the beam (“reference beam) is directed to a stationary plane mirror or cube corner retro-reflector attached to the beam splitter. The measurement portion of the beam (“measurement beam”) is directed to a movable plane mirror or cube corner retro-reflector. Typically, both retro-reflectors or plane mirrors are positioned and mounted to redirect the reference and measurement beams so that they recombine at the splitting interface of the beam splitter and are next directed to a suitable detector for measurement of the phase angle between the reference and measurement beams (from which is determined the relative displacement between the beam splitter and the movable cube corner retro-reflector). Incorporated into the rear face of the beam splitter is a single quarter wave plate, which reflects and changes the polarization state of beams impinging upon it. Such beams are nominally reflected from the reference mirror quarter wave plate parallel to one another. The single quarter wave plate serving as a reference mirror for both beams typically exhibits some degree of concavity, convexity or other type of geometric imperfection along its reflective surface, however, which leads to output tilt error or RBD, where the center normal vectors of the two beams are quasi-convergent.
What is needed is a monolithic interferometer that minimizes RBD and tilt, and that does so in an economic and practical manner.